1. Field of the Invention
The present invention relates to a progressive power lens used to assist an accommodation power of an eye.
2. Related Background Art
In general, when a person reaches his/her late forties, an accommodation power of his/her eyes becomes weak and he/she finds it difficult to see a near point clearly. Namely, his/her eyes become presbyopic. As spectacles for assisting an accommodation power of presbyopic eyes, progressive power spectacle lenses (hereinafter referred to as the "progressive power lenses") have been used widely. Such a progressive power lens has a distance vision correcting region (referred to as a "portion for distance vision" hereinafter), a near vision correcting region (referred to as a "portion for near vision" hereinafter) and a progressive region (referred to as an "intermediate portion" hereinafter) located between the above two portions in which a refracting power is continuously changed. When worn by a user, the portion for distance vision becomes an upper portion while the portion for near vision becomes a lower portion. In the present invention, it is to be noted that "upper", "lower", "horizontal" and "vertical" are used to refer to positions of the progressive power lens in the state that it is actually worn by a user. For example, the lower portion of the portion for distance vision is a position within the portion for distance vision close to the intermediate portion.
FIG. 1 shows divisions of regions of a progressive power lens formed symmetrically. The progressive power lens has a portion F for distance vision, a portion N for near vision and an intermediate portion P located between those two portions F and N in which a refracting power is continuously changed. As for the shape of a lens surface, the intersecting curve MM' of the object-side lens surface and a cross section along a meridian line passing through approximately the center of the lens surface and extending vertically from top to bottom is used as a reference line for expressing specifications such as the additional power of the lens, and is also used as an important reference line in the lens design. In the thus designed progressive power lens, the center OF of the portion F for distance vision, an eye point E for distance vision and the center (eye point) ON of the portion N for near vision are located on the center line MM'.
Further, as shown in FIG. 2, in consideration of the fact that the portion N for near vision comes close to a nasal side when worn by the user, other progressive power lenses have been proposed in which a portion N for near vision is arranged asymmetrically (hereinafter referred to as a "asymmetrical type progressive power lens").
Also, in such an asymmetrical type progressive power lens, a center line MM' consisting of the intersecting curve of an object-side lens surface and a cross section passing through the center OF of a portion F for distance vision, a geometric center OG of the lens surface and the center ON of a portion N for near vision is used as a reference line. In the present invention, these reference lines will be referred to as "the principal meridional curve" hereinafter.
The respective centers of the portion F for distance vision and the portion N for near vision are reference positions for measuring lens powers. A reference point for measuring the lens power of portion F for distance vision is called the center OF for distance vision while a reference point for measuring the lens power of portion N for near vision is called the center ON for near vision. In general, the center ON for near vision coincides with the eye point for near vision.
In progressive power lenses, additional refracting powers are imparted continuously on the principal meridional curve MM' from the center OF for distance vision toward the center ON for near vision. The value obtained by subtracting the power imparted to the center OF for distance vision from the maximum power imparted to the center ON for near vision is called the additional power.
Progressive power lenses in which clear vision areas of the portion F for distance vision, the intermediate portion P and the portion N for near vision are wide and fluctuation and distortion of an image are small are ideal progressive power lenses.
However, the requirements of wider clear vision areas are contradictory to the requirements of reduction of swim and distortion of an image and both sets of requirements cannot be satisfied completely, as will be discussed in detail later. In order to obtain an ideal progressive power lens that is easy to use, as well as comfortable, it is necessary to select parameters regarding lens characteristics and therefore a lens design idea is concerned. Presently, progressive power lenses with various characteristics are manufactured.
As parameters for designing, basically, there are maximum and minimum principal curvatures of respective points on the lens refracting surface, the distribution thereof, the gradients of the normal lines at the respective points on the lens refracting surface, and the directions thereof.
As the result of arranging these parameters on the lens surface, lens characteristics such as widths of the clear vision areas of the portion for distance vision, the intermediate portion and the portion for near vision, the length of progressive power range (the intermediate portion) along the principal meridional curve, the convergence of the portion for near vision, the distribution of mean powers, the gradient thereof, the distribution of the astigmatic differences, the gradient of the powers thereof, the direction of the axis of astigmatism, the distribution of prism power, the gradient of the prism degrees and the basal direction thereof, and distortion are imparted.
Further, characteristics of the progressive power lens as a spectacle lens is determined by the outline of the lens, the lens aperture in terms of forming the lens thinner and lighter, specifications of the material (refractive index, Abbe number, specific gravity, etc. ), the presence or absence of the prismatic thinning, the amount thereof, and the method of treating the lens surface.
In the above factors, the length of the intermediate portion, and the widths of the clear vision areas of the portion for distance vision, the intermediate portion and the portion for near vision are the factors greatly affecting the determination of the characteristics of the progressive power lens. It can be said that the characteristics of the progressive power lens are determined basically by these two factors.
For example, when a portion for distance vision and a portion for near vision are formed so as to have wide clear vision areas and are coupled by a progressive power range (intermediate portion), distortion of the curved surface is confined in a narrow area on the lens surface with a high density due to the provision of the intermediate portion. As a result, although the clear vision areas of the respective portions can be widened, lens aberrations, in particular, the astigmatic difference is concentrated on side areas of the intermediate portion. Due to the concentrated aberrations, blurring and distortion of an image occur in the side areas of the intermediate portion. Therefore, when a user wears spectacles formed of these progressive power lenses and shifts his eyes laterally, he perceives the distortion of an image as swim of the image and has an unpleasant feeling.
In this progressive power lens, although the clear vision areas of the portion for distance vision, the intermediate portion and the portion for near vision are enlarged, its evaluation is low and it cannot be said that the lens is easy to use and appropriate for practical use in that blurring, swim and distortion occur largely in the side areas of the intermediate portion where aberrations are liable to be concentrated.
Early progressive power lenses were of an aberration concentrated type in which aberrations were concentrated on side areas of an intermediate portion.
Also, when the length of an intermediate portion is shortened, the gradient of the additional power becomes steep. Therefore, aberrations are concentrated on the side areas of the intermediate portion.
According to Minkwitz's low described in Optica Acta (Volume 10, No. 3, July 1963), when the umbilical point form (the spherical shape becomes such that two principal curvatures respectively in the direction along the principal meridional curve and the direction perpendicular thereto are equal) is taken, the astigmatic difference on the lens surface is increased at a rate of double the surface refracting power on the umbilical point like principal meridional curve in the direction perpendicular to the principal meridional curve, so that a clear vision area cannot be enlarged.
In general, as the length of an intermediate portion is made large, swim, distortion and blurring of an image on side areas of a lens are lessened. However, due to the limitation in the angle of the rotation of the eye, the visual sight cannot be shifted with ease between the portion for distance vision and the portion for near vision. As a result, the user tends to use the portions for near vision for a long time. Therefore, the progressive power lens with the elongated intermediate portion is not suitable for practical use too.
However, if the visual range is limited such as from the portion for distance vision to the intermediate portion or from the intermediate portion to the portion for near vision, or the purpose of use is limited, the progressive power lens with the elongated intermediate portion can be used sufficiently.
The prime object in designing a progressive power lens is shifted from the conventional aberration concentrated type to an aberration distributed type in which aberrations are distributed to respective portions. Namely, it is required to form a progressive power lens to be the aberration distributed type to reduce swim and distortion of an image, to secure clear vision areas of a portion for distance vision, an intermediate portion and a portion for near vision widely. Also, in the lens designing, it is required to reduce swim and distortion of an image and to optimally secure clear vision areas of the respective portions by weighting parameters and carrying out the selection of the parameters, and arranging and unifying the selected parameters on the lens surface.
For optimizing the arrangement of refracting powers on a lens surface, a progressive power lens has been proposed in Japanese Patent Laid-Open Application No. 1-221721. In this progressive power lens, regarding the mean surface refracting power Q and the Gauss curvature G, a parameter consisting of its difference .DELTA.q (i.e., .DELTA.q=Q-G) is set in respective points forming side areas of the lens refracting surface in which the astigmatic difference is above 0.5 diopters. The parameter .DELTA.q represents the distribution of the astigmatic differences of the lens refracting surface, i.e., the amounts of the astigmatic differences and the gradient of the powers of the astigmatic differences. The arrangement of refracting powers on the side areas of the lens is optimized such that the value of the parameter .DELTA.q is within a range defined by a function of the reference average refracting power P.sub.B on the portion for distance vision (surface refracting power on the center for distance vision).
The invention disclosed in the above publication defines the condition of a curved surface constituting a lens refracting surface capable of reducing aberrations to be concentrated on side areas of a lens, in particular, side areas of an intermediate portion thereby to reduce swim and distortion of an image and the additional power A.sub.D.
Thus swim and distortion among visual characteristics can be reduced in the prior art to some extent. However, improvement of blurring (imaging defect) is performed to some extent, but is not yet sufficient.
In the above conventional progressive power lenses, visual performance is achieved to some extent but not sufficiently for practical use.